1. Technical Field
The present invention relates to an AC signal generating apparatus which generates, with a resolving power of about 1 ppm, a constant-voltage or constant-current AC signal of a frequency ranging between 10 and 40 MHz, for instance.
2. Description of the Related Art
FIG. 1 shows a conventional constant-voltage AC signal generating apparatus, in which an AC signal of a set voltage Va is generated by an AC signal source 11 and is supplied to a differential amplifier 12, its output is provided via an output terminal 13 to a load 14 and the voltage of the load 14 is negatively fed back to the differential amplifier 12 via a terminal 15 to ensure the application of the set voltage Va to the load 14.
FIG. 2 shows another prior art example. An AC signal from the AC signal source 11 is applied to an inverting input end of an operational amplifier 17 via a variable resistance element 16, for example, a heat sensitive resistance element such as a thermistor, a photosensitive resistance element such as a CdS, or a magnetic resistance element. The operational amplifier 17 has its non-inverting input end ground and its inverting input end and output end interconnected via a negative feedback resistor 18. The output of the operational amplifier 17 is provided via the output terminal 13 to the load 14, a voltage Vi which is applied to the load 14 is supplied via the terminal 15 to an output detector 19. In the output detector 19 the absolute value .vertline.Vi.vertline. of the load voltage Vi is obtained and its mean value .vertline.Vi.vertline. is provided to a comparator 21 for comparison with a reference voltage V.sub.E. The comparator output is used to control the resistance value of the variable resistance element 16 so that the mean value of the load voltage (amplitude) may be equal to the reference voltage V.sub.E.
In the conventional system of FIG. 1, since the input side of the differential amplifier 12 does not go into a virtually-grounded state, distortion is large and varies with a change in the operating point. Furthermore, when the differential amplifier 12 and the load 14 are spaced a long distance apart, there is a possibility that a phase delay of signal in the path from the output terminal 13 of the differential amplifier 12 to its inverting input end increases, providing a positive feedback to cause oscillation of the differential amplifier 12. Hence, this apparatus cannot be used under high frequency conditions.
The apparatus shown in FIG. 2 is of the type that controls the resistance value of the variable resistance element 16. Since the variable resistance element 16 has a nonlinear input voltage-output current characteristic, signal distortion is as large as about -40 dB of the fundamental wave.